1. Field of the Invention
The present invention relates to a rare earth permanent magnet which is useful not only for various electric and electronic devices, but also for motors installed in automotive vehicles. More particularly it relates to a high-performance magnet of the type containing cerium as the rare earth element, with the typical ratio of Ce to other elements being 1:5.
2. Description of the Prior Art
Among numerous kinds of rare earth permanent magnets, Ce-containing magnets and Sm-containing magnets which are basically composed of intermetallic compounds, CeCo.sub.5 and SmCo.sub.5, respectively, are widely employed. (These are conventionally called 1/5 magnets.) The use of the Sm-containing 1/5 magnets in the fields of electric devices and electronics has sharply increased, despite the fact that these magnets contain expensive samarium and cobalt, because the Sm-containing magnets are capable of exhibiting very high magnetic characteristics. For example, the maximum energy products (BH).sub.max of some Sm-containing 1/5 magnets are about 20 MG.Oe, which is several times as high as those of conventional ferrite and Alnico magnets. A permanent magnet formed from SmCo.sub.5 and Sm.sub.2 Co.sub.7 which has an energy product as high as 15 MG.Oe and up to 20 MG.Oe is disclosed in U.S. Pat. No. 4,075,042.
However, motors for automotive vehicles and domestic electric appliances do not require magnets having a performance as high as a Sm-containing 1/5 magnet. A magnet having a maximum energy product of 10 MG.Oe would be good enough, and therefore the Ce-containing 1/5 magnets are more suitable for those motors. The existing Ce-containing 1/5 magnets, however, are too expensive when compared with non-rare earth permanent magnets used in similar applications. An object of researchers in the field is to reduce the content of expensive cobalt in the Ce-containing magnets without affecting the magnetic properties.
In this regard, the following magnets have been disclosed: (i) Ce(Co.sub.0.72 Fe.sub.0.14 Cu.sub.0.14).sub.5 in "IEEE Trans. Mag Mag", 10,560, (1972); and (ii) Ce(Co.sub.a Cu.sub.b Fe.sub.c Zr.sub.d).sub.z wherein the content of Fe is from 0.03 to 0.2 in Japanese Kokai (Sho) 62-51484. As can be surmised from these disclosures, a Ce-containing 1/5 magnet fails to maintain the desired magnetic characteristics when its Fe content exceeds 0.2.
However, in the case of Sm-containing magnets, it is known that high magnetic characteristics are maintained even when the Fe content exceeds 0.2 or approaches 0.3 (ref. J. Appl. Phys. 52(3)2517, 1981). These Sm-containing magnets wherein the Fe content is from 0.2 to 0.3 are conventionally called "2/17 magnets" because the ratio of Sm content to the others is roughly 2:17. An object of the present invention is to provide Ce-containing magnets which has high Fe contents and exhibits the desired level of magnetic properties. The Sm-containing 2/17 magnets do not suggest how to do this since the solid solubility of Fe in Sm-containing 2/17 magnets is quite different from that in Ce-containing 1/5 magnets.
Magnets which exhibit a maximum energy product of 10 MG.Oe or higher include NdFeB magnets and SmCo plastic magnets. Although the former contain materials which give rise to high magnetic properties, the stability of its magnetic properties with respect to temperature changes is poor. Also, the existence of neodymium renders the magnet so vulnerable to oxidation (rusting) that it must be coated, and consequently the overall cost becomes as high as that of Sm-containing 2/17 magnets. The SmCo plastic magnet is favored because it can be formed in arbitrary shapes and it needs no finishing treatments such as coating. However, since a magnetic powder containing 90 wt % or more of samarium must be employed, the material cost becomes very high.